Circadian rhythms are produced by the body's 24-hour internal clock that regulates biological processes such as sleep, wakefulness, hormone release, digestion and body temperature.

When our circadian rhythms are disrupted, it can affect cardiovascular, kidney and metabolic health.

The American Heart Association (AHA) recently issued a new scientific statement that outlines how these disruptions can increase the risk of heart disease and other outcomes such as obesity, type 2 diabetes and high blood pressure, or hypertension.

Light is the primary cue that synchronizes the central circadian clock in the brain. Early morning exposure to light helps reinforce healthy circadian rhythms.

“The goal of the statement is to emphasize that the timing of behaviors such as meals, sleep and exercise and the timing of exposure to light can influence our circadian rhythms,” Kristen Knutson, volunteer chair of the AHA's scientific statement writing committee, told TheDoctor.

We have clocks throughout our body, she explained. We have the central clock in our brain and clocks in our tissues and organs. These clocks should be synchronized, both internally and externally, with what is going on outside.

People need to understand the role that timing and consistency play in health. “If you are jumping around the clock with your meals and your sleep, your internal clock gets confused about what time it is because it uses that information to synchronize,” Knutson added.

These are the key take-home messages from the statement:

  • Sleep timing and consistency are as important as sleep duration. An irregular sleep schedule can disrupt circadian rhythms. Sleep is a key measure for improving and maintaining heart health in the AHA's Life's Essential 8.
  • Light exposure timing is a therapeutic tool. Light is the primary cue that synchronizes the central circadian clock in the brain. Early morning exposure to light helps reinforce healthy circadian rhythms. Night time exposure to artificial light, such as the light from computer screens or smartphones, can suppress melatonin and delay sleep onset.
  • Meal timing affects metabolic health. It's not just about caloric content. Late night meals or irregular mealtimes can throw off the circadian clocks in organs such as the liver and pancreas, and cause fluctuations in blood sugar levels. These fluctuations can lead to weight gain. Studies have found that eating earlier in the day can reduce the risk of type 2 diabetes and improve cardiometabolic health.

    • Late night meals or irregular mealtimes can throw off the circadian clocks in organs such as the liver and pancreas, and cause fluctuations in blood sugar levels.

  • The timing of physical activity helps regulate circadian rhythms. Morning or afternoon workouts can help regulate circadian rhythms, but evening workouts can disrupt them.
  • Shift work can disrupt circadian rhythms. The disruption of circadian rhythms by the irregular mealtimes and inconsistent sleep patterns associated with shift work is an established risk factor for cardiovascular disease.
  • A person's unique internal clock, or chronotype, should guide interventions. Some people are early birds while others are night owls.

A person's chronotype — their tendency to sleep at a particular time during a 24-hour period — affects how a person responds to light, meals and exercise. Tailoring health interventions to a person's unique chronotype can improve their effectiveness and regulate circadian rhythms, and potentially lead to healthier outcomes. “Ideally, the timing of these behaviors are relevant to whatever your internal clock is,” said Knutson, a professor of neurology at the Northwestern University Feinberg School of Medicine.

The AHA scientific statement is published in Circulation.